1. Field of the Invention
This invention relates to a method of making a light emitting element including a semiconductor layer represented by: AlXInYGa1−X−YN (0≦X≦1, 0≦Y≦1, 0≦X+Y≦1).
2. Description of the Related Art
Conventional light emitting elements include LED (Light Emitting Diode) elements with a GaN based semiconductor layer. Generally, the LED elements are formed by sequentially growing, through AlN buffer layer on a sapphire substrate, an n-GaN layer, an MQW (Multiple Quantum Well) layer and a p-GaN layer.
When a current is fed between the n-GaN layer and the p-GaN layer, the MQW layer emits light.
If there is a significant difference in refractive index between the semiconductor layer and a material for sealing the semiconductor layer, total reflection easily occurs in the LED element so that the light extraction efficiency lowers.
In order to solve this problem, it is tried to process the surface of the semiconductor layer in nano-order. However, the process equipment grows in size and the surface of all the LED elements produced is difficult to process at good stability and reproducibility in case of processing a wafer with a relatively large area or a number of the LED elements arrayed.
JP-A-2005-223362 discloses a method of making an LED element that an end surface of the LED element is etched to form a nonspecular surface by using KOH aqueous solution. The LED element of JP-A-2005-223362 uses a GaN substrate as a growth substrate instead of the sapphire substrate, and the nonspecular surface is formed on the end surface as well as on the N-surface of the GaN substrate by etching.
JP-A-2005-019835 discloses a nitride semiconductor laser element that GaN is wet-etched using alkali aqueous solution such as TMAH (Tetra-Methyl-Ammonium-Hydroxide). In the nitride semiconductor laser element of JP-A-2005-019835, a current-confining structure is formed by etching AlGaN with a high Al ratio by the TMAH aqueous solution as an alkali developing solution.
However, in the above methods for making the LED element, the etching process is conducted in an environment using a lighting system etc., so that light with a wavelength region higher than the band gap energy enters into the semiconductor layer during the etching process.
Thus, pairs of electron and hole are generated on the surface of the semiconductor layer by light excitation, and the hole may break an atomic binding in the crystal of the semiconductor layer, so that erosion nonuniformity due to electric potential imbalance cannot be prevented.
As a result, as shown in FIG. 17, there is the problem that the shape and distribution of the concavity and convexity formed on the roughened surface of the semiconductor layer is nonuniform along with the reduction of the processing accuracy, so that it is not possible to sufficiently improve the light extraction efficiency by accurately conducting the fine processing.
JP-A-2005-019835 mentions that the AlGaN layer with a high Al ratio can be etched by the TMAH aqueous solution, but the AlGaN layer with a low Al ratio or GaN layer cannot be etched thereby so that such a layer functions as an etching stop layer.